<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>导航栏下划线跟随</title>
  <style>
    .nav{
      width: 1000px;
      height: 300px;
      margin: 0 auto;
    }
    ul{
      list-style: none;
      display: flex;
    }
    li {
      padding: 5px 10px;
      border-bottom: 0px solid #000;
      cursor: pointer;
      position: relative;
    }
    li::before {
      content: "";
      position: absolute;
      top: 0;
      left: 100%;
      width: 0;
      height: 100%;
      border-bottom: 2px solid #000;
      transition: 0.2s all linear;
    }
    li:hover::before {
      width: 100%;
      top: 0;
      left: 0;
      transition-delay: 0.1s;
      border-bottom-color: #000;
      z-index: -1;
    }
    li:hover ~ li::before {
      left: 0;
    }
    li:active {
      background: #000;
      color: #fff;
    }

  </style>
</head>
<body>
  <div class="nav">
    <ul>
      <li>不可思议的CSS</li>
      <li>导航栏</li>
      <li>光标小下划线跟随</li>
      <li>PURE CSS</li>
      <li>Nav Underline</li>
    </ul>
  </div>
</body>
</html>
